The present invention relates to an optical scanning device and more particularly to an optical scanning device for scanning beams from a plurality of light emitting sources over a scanned face.
Hitherto, an image formation system such as a laser beam printer for irradiating a rotating polygon mirror with a laser beam from a single light source and applying the reflected beam from the rotating polygon mirror to the charged surface of a photosensitive body moving at constant speed has been known. In such an image formation system, the laser beam from the light source is modulated in response to the digital image data of the image to be output and as the rotating polygon mirror rotates, the laser beam is scanned in the direction perpendicular to the move direction of the photosensitive body surface, thus an electrostatic latent image is formed on the photosensitive body and is developed to a visible toner image.
To provide an output image of high quality at high speed by the image formation system, it becomes necessary to scan a high-speed and high power laser beam.
However, there are limitations of the number of revolutions of the rotating polygon mirror and the luminous energy of the laser beam emitted from a single light source. To solve this problem, Examined Japanese Patent Publication No. Hei 1-52728 proposed a related art for using a laser diode array (MSLD: Multispot laser diode) comprising a number of light emitting sources as a light source and scanning a high power laser beam. In this art, MSLD is used for speeding up and the focal length of a collimator lens is made short as much as possible and the laser beam from the MSLD is taken as scan beam as much as possible for providing high power output. In other words, image formation magnification .beta.T along the main scanning direction is made great.
That is, as shown in FIG. 10, an image formation system 90 is designed so that light sources 92A and 92B and a collimator lens 94 are brought close to each other. Letting the focal length of a scan lens 96 be f and the focal length of the collimator lens 94 be f.sub.c, image formation magnification .beta.T=f/f.sub.c, thus the smaller the focal length f.sub.c, the greater the image formation magnification .beta.T.
However, if the MSLD and the collimator lens are brought close to each other in placement as described above, when the spacing between the MSLD and the collimator lens changes due to temperature change, vibration, etc., an defocus or dealignment phenomenon easily occurs on a photosensitive body and it is feared that the quality of the formed image may be degraded.
Generally, the beam diameter in the scanning direction is made smaller than that in the sub scanning direction to provide a high-quality image, thus the focus depth in the scanning direction is smaller than that in the sub scanning direction; the defocus phenomenon easily occurs.